Question of the Month

Answer:

The Earth spins on its "axis". This axis is an imaginary line running through the Earth. If you
were to be high above the Earth, looking straight down along the axis, all the points on Earth would
appear to move in circles around the axis. If you followed this axis out into space from the northern
hemisphere on Earth, it would point toward a particular star in the sky. We call that star the
"North Star" since it sits in the direction that the spin axis from the northern hemisphere of Earth
points.

At present, the star known as Polaris is the North Star. However, Polaris has not always
been the North Star and will not always be the North Star. To understand that, we need to look at
how the Earth spins on its axis.

The spin axis of the Earth undergoes a motion called precession. If you have ever watched a
spinning top, you know that its spin axis tends to stay pointed in the same direction. However,
if you give it a slight nudge, the axis will start to change its direction, and its motion traces
out a cone. This changing of direction of the spin axis is called precession. So what gave the
Earth the "nudge" it needed to start precessing? The Earth bulges out at its equator, and the
gravitational attraction of the Moon and Sun on the bulge provided the "nudge" which made the
Earth precess. It was the ancient Greek astronomer and mathematician Hipparchus who first
estimated the precession
of the Earth's axis around 130 B.C. The period of precession is about 26,000 years. In other words,
it takes 26,000 years for the axis to trace out the cone one complete time.

You can see precession of the spin axis in a spinning top

Earth's spin axis also precesses. It takes 26,000 years to go around once!

So now you can see why Polaris will not always be aligned with the north spin axis of the Earth -
because that axis is slowly changing the direction in which it points! Right now, the
Earth's rotation axis happens to be pointing almost exactly at Polaris. But in the year
3000 B.C., the North Star was a star called Thuban (also known as Alpha Draconis), and in about
13,000 years from now the precession of the rotation axis will mean that the bright star Vega
will be the North Star. Don't feel bad for Polaris, however, because in 26,000 more years it
will once again be the Pole Star!

By the way, there is not currently a star in the direction of the southern hemisphere spin axis.
So we do not now have a "South Star".